In a WDM (wavelength division multiplex) optical communication system, multiple component wavelengths of light each carry a communication signal. Each of the multiple component wavelengths of light form a WDM channel. An optical add-drop multiplexer (OADM) is used for managing the WDM signals that are carried from location to location using the WDM channels. At a particular location, the WDM signal within each WDM channel is either passed for transmission to another location, is added for transmission or is dropped for local distribution. As a component signal or channel is dropped, the component signal or channel corresponding to the dropped component signal or dropped channel is free to accept an added component signal or channel. The new added component signal is uploaded into the WDM signal at the same wavelength as the dropped component signal. Maintaining an active signal in each channel maximizes total bandwidth.
The purpose of wavelength division multiplexing is to carry multiple signals over the same medium at the same time. To accomplish this, a number of channels are used. However, different signals may need to be transmitted to different locations at any time. Thus, if a given component signal is transmitted for the predetermined distance, that component signal is dropped and another component signal is added it is place, thereby maximizing the total bandwidth.
FIG. 1 schematically illustrates a functional diagram of an optical add and drop multiplexer 10 for carrying multiple signals over the same medium. Shown in FIG. 1 is an optical add and drop multiplexer (OADM) 10 having two WDM input signals 12, 14 and two WDM output signals 16, 18. Of the two WDM input signals 12, 14, the WDM input signal 12 includes within, component signals or WDM channels 1, 2, 3, 4 and 5. In addition, the WDM input signal 14, also called an WDM add signal 14, shown in FIG. 1, includes channels or component signals 1′, 2′, 3′, 4′ and 5′, some of which are to be added to the WDM input signal 12. Of the two WDM output signals 16, 18, the WDM output signal 16 includes WDM channels or component signals that correspond to the combination of the multiplexed signals of the WDM input signals 12 and 14.
In FIG. 1, the WDM input signal 12 contains component signals 1, 2, 3, 4 and 5, whereas the WDM add signal 14 contains three component signals, 2′, 4′ and 5′ which are to be added to the WDM input signal 12. The three component signals to be added, 2′, 4′ and 5′ contain local information which are uploaded by the OADM 10. The two WDM input signals 12, 14 are multiplexed, whereby the OADM 10 adds the three component signals 2′, 4′ and 5′ from the WDM add signal 14 and drops the corresponding three component signals 2, 4 and 5 from the WDM input signal 12. The three component signals 2, 4 and 5 are then dropped for local distribution at a given location, which may be the same or different location from where the added component signals 2′, 4′ and 5′ are uploaded. Component signals 2′, 4′ and 5′ are modulated at the same wavelength as component signals 2, 4 and 5. The added component signals 2′, 4′ and 5′ are also interlaced with the two passed component signals 1 and 3 to form a WDM output signal containing channels 1, 2′, 3, 4′ and 5′. This process is referred to as an add/drop function.
To perform the add/drop function, the component signals within the WDM signal must be isolated. Conventionally, a multiplexer/de-multiplexer is used to separate the component signals and an array of waveguides are used to direct each component signal to a desired location. Waveguides tend to be expensive, they are typically delicate to set-up and maintain, and often use extensive thermal management.
Once the component signals are isolated MEMS (MicroElectroMechanical System) mirrors or tilting mirrors are often used to reflect each component signal in a predetermined direction. The component signal is either passed or dropped depending on the predetermined direction. To predetermine a direction, the mirrors are moved or rotated using some type of mechanical means, for example a piezoelectric or pico-motor. Such mechanical movement produces mirror movements that may be less precise than desired. Mechanical movement also limits the speed by which the mirrors can be moved, and thus limits the speed by which the channels can be added/dropped.
An alternative means to perform the add/drop function is to use a Mach-Zehnder interferometer for each component signal. The Mach-Zehnder interferometer is an amplitude splitting device consisting of two beam splitters and two totally reflecting mirrors. The component signal is split into two portions and each portion is directed along separate optical paths. The two portions are eventually recombined. When recombined the two portions either constructively interfere or destructively interfere depending on whether or not the component signal is to be passed or dropped, respectively. The type of interference is determined by the phase difference between the two portions upon recombination. Changing the optical path lengths of one or both of the two portions can alter the phase difference. A difference between the optical path lengths can be introduced by a slight tilt of one of the beam splitters. To tilt the beam splitter though uses some type of mechanical means, which once again limits speed and precision. Also, since the two paths are separated, the Mach-Zehnder interferometer is relatively difficult to align and maintain. Mach-Zehnder interferometers are also expensive and often utilize extensive thermal management.
What is also needed is a method of adding and dropping channels within a WDM signal that is less expensive and simpler to implement and maintain than conventional optical add/drop multiplexers, and that increases speed and improves precision.